A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in device manufacturing processes, such as in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which may alternatively be referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. A single substrate may include a network of adjacent target portions that are successively patterned. Lithographic apparatus may include a stepper and/or scanner. A stepper may be configured such that each target portion is irradiated by exposing an entire pattern onto the target portion at one time. A scanner may be configured such that each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor a device manufacturing process such as a lithographic process, one or more parameters of the patterned substrate (and therefore of any aspect of the device manufacturing process that affects the patterned substrate) may be measured. The one or more parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and/or critical dimension (e.g. linewidth, or the like) of developed photosensitive resist and/or etched product features. One or more of the parameters may include feature heights and/or feature pitches. These measurements may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing properties of the beam before and after it has been reflected, diffracted and/or scattered by the substrate, one or more properties of the substrate may be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with one or more known substrate properties or data calculated in real time from a model of the scattering structure.